/Engine/Source/Runtime/SlateCore/Public/Styling/CoreStyle.h
Buttons already have a built in (4., 2.) padding - adding to that a little.
namespace CoreStyleConstants { const [FMargin](API\Runtime\SlateCore\Layout\FMargin) CoreStyleConstants::ButtonMargins ( 12. f, 1. 5f, 12. f, 1. 5f ) }